We will study genome structure and expression in a simple multicellular eucaryote as a function of its differentiation and development. The free-living nematode, Caenorhabditis elegans, is the experimental organism. We will examine the DNA complexity by reassociation kinetics and by the patterns of hybridization of cloned fragments to total DNA (Southern technique), analyze the arrangement of the middle and highly repetitive DNA's, determine whether there are DNA sequence rearrangements during somatic differentiation by comparing hybridization patterns of cloned fragments to somatic and germ line DNA's, determine whether different strains contain DNA sequence differences that can be used as phenotypes for genetic mapping, and biochemically analyze the structure of the ribosomal genes and genetically map them. We will isolate the actin genes using recombinant DNA techniques, measure the number of actin genes, map the actin genes using DNA sequence differences between strains as phenotypes, measure the stability of the actin DNA sequences during somatic differentiation, and use cloned actin DNA sequences to assay by in situ hybridization the presence and specific location of actin mRNA's in specific known cells during development.